1. Field of the Art
The present invention relates to a method and an apparatus for controlling an electromagnetic powder clutch for use on an automotive vehicle.
2. Related Art Statement
In the automotive vehicle art, it is recognized that the fuel economy of the vehicle is improved when the engine is operated at a relatively low speed and under a relatively high load. This low speed-high load operating condition is established by controlling a speed ratio of a transmission of the vehicle or selecting a suitable speed range of the transmission. In the low speed-high load operating condition, however, fluctuation in output torque of the engine tends to be large due to a comparatively long ignition interval of the engine. The resulting vibrations and noises felt by the vehicle operator are accordingly increased, and driving comfort is deteriorated. These inconveniences are considered a barrier to practicing the above-indicated low speed-high load control method, which is otherwise effective to enable the engine to provide a sufficient average output to drive the vehicle with a high level of fuel economy.
It is known to use an electromagnetic powder clutch which is disposed between an engine and a transmission to transmit the output of the engine to the transmission. An example of an arrangement incorporating such an electromagnetic powder clutch is disclosed in Japanese Patent Application, laid open in 1983 under Publication No. 58-657. In the disclosed arrangement, the engagement of the powder clutch is controlled so as to absorb variations in the output torque of the engine by positively giving the powder clutch a small amount of slip such that the output speed of the powder clutch is lower than the input speed by a value equal to the slip speed of the clutch that is, the difference between the input and output speeds of the clutch. This slip speed is selected to be larger than an average variation in the input speed of the clutch caused by variations in the output torque of the engine. Thus, the disclosed arrangement is useful for improving the driving comfort of the vehicle while the engine is operated at low speeds.
However, the method of controlling the powder clutch as disclosed in the document identified above is not satisfactory in terms of operating response of the powder clutch to engine torque variation. Specifically, the torque to be transmitted by the powder clutch should be finely controlled so that the slip speed of the powder clutch falls within a very small range, for example, from 10 to 30 rpm. To control the engagement of the powder clutch for stable and accurate fine adjustment of its slip, the magnitude of a signal for adjusting the amount of slip of the powder clutch should be varied slightly each time a control cycle is repeated. In other words, the slip adjustment value updated in each control cycle tends to be very small. Consequently, the signal for adjusting the amount of slip of the powder clutch may not be sufficiently responsive to variations in the output torque of the engine when the engine torque is varied at a high rate, such as during a rapid increase in the opening angle of a throttle valve. In such conditions, the amount of slip of the powder clutch is likely to be excessive, and the driving comfort and fuel economy of the vehicle lowered.